Names: _________________________
____________________________
Experiment 12H Part C
(Due Thursday 3-24)
This document includes everything you need for the lab report. Show your work clearly and put
the correct unit on EVERY number!
READ THE PROCEDURE IN THE LAB TO FAMILIARIZE YOURSELF WITH THIS
EXPERIMENT!
In this experiment, you measure the equilibrium constant for the following reaction at various
temperatures.
Fe3+ (aq) + SCN− (aq) = FeSCN2+ (aq)
A plot of ln Keq vs. 1/T allows you to determine Ho, So, and Go for the reaction.
The reactants have no color and the product is red so the amount of product present at equilibrium is
measured by reading the absorbance at 460 nm.
Data for this experiment is given below.
DATA:
= 4925 M-1cm-1
l = 1.00 cm
[Fe3+]init = 1.00 x 10-2 M
[SCN−]init = 1.00 x 10-4 M
Corrected
T (oC)
5.9
8.9
12.8
15.9
19.6
22.1
A
0.309
0.305
0.300
0.296
0.292
0.288
[FeSCN2+]eq
Keq
T (K)
1/T (K-1)
ln Keq
The temperature was corrected based on the measured temperature of ice-water. Using Beer’s law
(A = cl), you can calculate the equilibrium concentration of the product. (The values for  and l are
given above.) From the given initial concentrations of reactants and the measured equilibrium
concentration of the product, you can set up an ICE table and calculate the equilibrium constant at each
temperature.
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1. Reproduce and complete the table on the previous page in Excel. Use Excel formulas to do all of
the calculations except for Keq. Use ICE tables to calculate Keq. Show your calculation of
[FeSCN2+]eq , the ICE table, and your calculation of Keq for 5.9oC below.
2. Based on how the equilibrium constant changes with temperature, determine whether the
reaction is endothermic or exothermic. Explain your answer.
3. Based on the nature of the reaction, does your answer to question 2 make sense? Explain.
(Consider bond breaking and bond forming.)
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4. Use Excel to make a graph of ln Keq vs. 1/T. Add a trend line and show the equation and R2
value.
5. Use the equation of the line to calculate Ho and So (see ws18.3). Does the sign of Ho match
your answer to question 2?
6. Calculate Go at 25oC. (See ws18.2. Assume that Ho and So are independent of temperature.)
What does the sign of Go tell you about this reaction?
7. Calculate the value of the equilibrium constant at 25oC (see ws18.3). Does your answer make
sense based on your measured values of Keq at lower temperatures? Does your answer make
sense based on the sign of Go?
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8. You can also calculate the equilibrium constant at 25oC using the equation of the line. Do this
and compare your answer to your answer to question 7.
9. Based on the balanced equation for the reaction, predict the sign of So. Explain your answer.
10. If this was a gas phase reaction, your prediction in question 9 would have been correct but it
turns out that this reaction actually has the opposite sign of So. (If you got the same sign as
your prediction, you’ve done something wrong!) Can you figure out why this is? [Hint:
Remember that all of the ions are hydrated (surrounded by water). What must be happening to
the water?]
11. You have determined the value of G at 25oC when Q = 1 (Go). Calculate G at 25oC when
Q = 200.0 (see ws18.3). Does the sign of G make sense based on your calculated value of Keq
at 25oC?
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